Genome Evolution of Color Vision in Humans and Primates
Ever wonder what truly compels fighting bulls to charge at that bright, red cape dangling in the center of the ring? Believe it or not, it has nothing to do with the color, but the movement of the cape. This is because bulls, like most other mammals, are essentially colorblind. More specifically, they only have two color-sensitive retina pigments, known as opsins, compared to our three (1). How do we see color? In mammals, the lens of the eye focuses light on a layer of rod and cone cells within the retina. These cells contain visual pigments known as opsins or rhodopsins. Rhodopsin, found in the rod cells, is the protein involved in low-light vision. For mammals that are trichromatic such as humans, apes, and Old World monkeys, each cone cell contains one of three types of opsins, each being sensitive to a different range ofwavelengths of light. These are typically referred to as red, green, and blue receptors. However, they are actually most sensitive to violet, green, and yellow-green light (2). Those who have only two different kinds of opsins, such as lemurs, are classified as dichromatic, seeing only blues and greens. Those who have only one kind of opsin, such as spider monkeys, are classified as monochromatic, seeing only black, white, and grey (3). Due to a rare fourth opsin gene, some vertebrates are able to see ultraviolet light as well as other colors that we are unable to detect. Most amphibians, reptiles. and birds, on the other hand, have also inherited trichromacy. However, many mammals remain at two opsins. This is most likely because some of the earliest mammals were nocturnal. This means they had virtually no need for color-sensitive opsins, especially since they only work in the daylight. Therefore, since they were never needed, they were never used, and eventually became a lost trait (1). In terms of genetics, one kind of opsin, the “blue” or short-wavelength-sensitive (SWS) type, is encoded by a gene on chromosome 7. The genes that encode both “green” or medium-wavelength-sensitive (MWS) opsins and “red” or long-wavelength-sensitive (LWS) opsins are found adjacent to each other on the X chromosome (2). Gene Mutation That being said, why do we have three opsins when most mammals have two? This is due to a mutation datingback to our primal ancestors. The MWS/LWS gene, coding for one of the two pigments found in most mammal eyes, was duplicated. Typically, extra gene copies degenerate as they acquire mutations. This mutated copy, however, resulted in a permanent ability to detect a separate spectrum of light, allowing us a trichromatic kind of vision (1). Usually, humans posses one copy of the LWS gene and multiple copies of the MWS gene. Because they are so closely arranged, this suggests that the many copies are a result of gene duplication. In this process, a single gene is duplicated and one of its copies accumulates mutations, eventually becoming a whole new gene that encodes for a protein with a different function. In this case, it is a red opsin. It is thought that about 40% of the human genome has evolved in this way (2). Sequence analysis has allowed scientists to determine that trichromacy in all Old World primates is dependent on separate X-linked MWS and LWS opsin genes, as opposed to one, that are organized into a head-to-tail tandem array flanked on the upstream side by a locus control region, which is critical for the expression of either gene. In Old World monkeys, it was found that the separate opsin genes diverged after allele duplication, while howler monkeys showed different opsin alleles that have risen and become fixed as different loci by duplication (4). Another kind of trichromacy, known as allelic trichromacy has revolutionized in a separate manner. Certain animals including New World monkeys have an autosomal S-opsin gene, yet only one X-linked opsin gene. However, this gene is polymorphic, meaning that it encodes different M to L opsins. As a result, heterozygous females are trichromatic, since they possess two different variations of the gene. Therefore, males, as well as homozygous females, are dichromatic (4). Ecological studies suggest some evolutionary advantage to trichromatic vision. Within a forest environment, this mutation makes it especially easy to find fresh leaves and fruit. For example, howler monkeys have a leaf-based diet, searching for fresh red-tinged leaves that are both nutritious and readily digestible to them (2). References 1. Le Page M. "Five classic examples of gene evolution". NewsScientist. 2009. 2. Bryk J. Evolution of colour vision in primates: case study. DNA to Darwin. 2011. 3. O'Neil D. Primate Color Vision. Palomar College. 2012. 4. Dulai K., von Dornum M., Mollon J., and Hunt D. "The Evolution of Trichromatic Color Vision by Opsin Gene Duplication in New World and Old World Primates". Genome Research. 1999.